Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical metrology processes are used at various steps during a semiconductor manufacturing process to perform measurements and detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of metrology based techniques including scatterometry, ellipsometry, and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
The porosity of films used in the manufacture of semiconductor devices significantly impacts device performance. Porous films are used as dielectric layers in the semiconductor fabrication process. In some examples, the dielectric constant of a film is reduced by increasing porosity. This results in reduced switching delay and improved device performance. There is a need to measure porosity quickly and in a non-destructive manner. In particular, total porosity, pore size, pore size distribution and pore sealing are parameters of interest for process monitoring and tool monitoring applications.
Currently, porosity measurements are most commonly performed using X-ray reflectivity and positron annihilation spectroscopy (PALS) techniques. Unfortunately, these techniques are unsuitable for production environments.
Ellipsometric porosimetry has been explored as a technique for measuring porosity based on spectroscopic ellipsometry (SE). This technique combines adsorption/desorption isotherms together with measurements of the index of refraction of liquid-filled pores and liquid-free pores. Unfortunately, these measurements are generally performed in a high-vacuum environment, which is suitable as a laboratory reference technique, but is undesirable for high-volume manufacturing. Exemplary techniques are described by A. Bourgeois et al., in “Description of the porosity of inhomogeneous porous low-k films using solvent adsorption studied by spectroscopic ellipsometry in the visible range,” Thin Solid Films 455-456, pp. 366-369 (2004), C. Negoro et al., “Nondestructive Characterization of a Series of Periodic Porous Silica Films by in situ Spectroscopic Ellipsometry in a Vapor Cell,” Jap. J. of Appl. Phys. Vol. 43 No. 4, pp. 1327-1329 (2004), and F. N. Dultsev, “Investigation of the microrporous structure of porous layers using ellipsometric adsorption porometry,” Thin Solid Films 458, pp. 137-142 (2004), the contents of each are incorporated herein by reference in their entirety.
U.S. Pat. No. 7,907,264, assigned to KLA-Tencor, Corporation, and incorporated herein by reference in its entirety, describes methods of estimating the porosity of a thin film based on measurements of the refractive index in different relative humidity conditions. The estimate of porosity is based on a correlation between the refractive index and film porosity. However, the correlation is established off-line by performing porosity measurements of a representative sample using a trusted measurement tool, such as an X-ray reflectometer or a PALS system.
Future metrology applications present challenges due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. Thus, methods and systems for improved measurements are desired.